Salvatore M. Oriti and Zachary D. Williams

Glenn Research Center, Cleveland, Ohio

Structural Dynamics Testing of 

Advanced Stirling Convertor Components

NASA/TM—2013-216544

October 2013

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Salvatore M. Oriti and Zachary D. Williams

Glenn Research Center, Cleveland, Ohio

Structural Dynamics Testing of 

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NASA/TM—2013-216544

October 2013

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NASA/TM—2013-216544 1 

Structural Dynamics Testing of 

Advanced Stirling Convertor Components 

Salvatore M. Oriti and Zachary D. Williams 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

NASA Glenn Research Center has been supporting the development of Stirling energy conversion for 

use in space. Lockheed Martin has been contracted by the Department of Energy to design and fabricate 

flight-unit Advanced Stirling Radioisotope Generators, which utilize Sunpower, Inc., free-piston 

Advanced Stirling Convertors. The engineering unit generator has demonstrated conversion efficiency in 

excess of 20 percent, offering a significant improvement over existing radioisotope-fueled power systems. 

NASA Glenn has been supporting the development of this generator by developing the convertors 

through a technology development contract with Sunpower, and conducting research and experiments in a 

multitude of areas, such as high-temperature material properties, organics testing, and convertor-level 

extended operation. Since the generator must undergo launch, several launch simulation tests have also 

been performed at the convertor level. The standard test sequence for launch vibration exposure has 

consisted of workmanship and flight acceptance levels. Together, these exposures simulate what a flight 

convertor will experience. Recently, two supplementary tests were added to the launch vibration 

simulation activity. First was a vibration durability test of the convertor, intended to quantify the effect of 

vibration levels up to qualification level in both the lateral and axial directions. Second was qualificationlevel vibration of several heater heads with small oxide inclusions in the material. The goal of this test 

was to ascertain the effect of the inclusions on launch survivability to determine if the heater heads were 

suitable for flight. 

Background 

NASA Glenn Research Center has been researching Stirling energy conversion technologies for 

several decades (Schreiber (2006)). Its potential for long life and high conversion efficiency has made it 

an attractive technology for space applications. The free-piston configuration enables long life with high 

reliability due to its elimination of wear mechanisms. This configuration is also useful because it permits 

direct conversion of thermal to electrical power within one assembly. Several configurations were 

studied, fabricated, and tested. However, dynamic energy conversion has yet to be used in a space power 

application. In 1999, NASA and the Department of Energy initiated development of a 100-We-class 

radioisotope-fueled generator that would use free-piston Stirling conversion. The goal was to develop a 

new power system for space science missions that would require less plutonium fuel than the status quo 

thermoelectric power systems. Lockheed Martin (LM) was later selected as the system integrator. In 

2006, the generator was given the title “Advanced Stirling Radioisotope Generator (ASRG),” when the 

project was redirected to use Sunpower’s Advanced Stirling Convertor (ASC) technology (Wong (2012)). 

The ASRG consists of two ASCs arranged in the dual-opposed configuration for dynamic balance. Each 

convertor is supplied one general purpose heat source (GPHS) module, located on the outer ends of the 

generator. A conductive flange connects the heat rejection zone of the convertor to the generator housing, 

which acts as a thermal radiator. The two convertors are mechanically coupled in the center via an 

interconnecting tube. 
NASA/TM—2013-216544 2 

As with any current radioisotope power system (RPS) design, the thermal energy from the fuel cannot 

be turned off. This means the convertors must be operating once the generator is fueled and throughout 

launch. The flight convertors will actually experience more than just a single exposure to launch 

vibration. There are four instances that a convertor will be exposed to random vibration: convertor 

workmanship, generator workmanship, generator flight acceptance, and launch. The workmanship 

vibration profile used for these tests is defined by NASA Standard (STD) 7001A “Payload Vibroacoustic 

Test Criteria.” The flight acceptance profile is defined by Jet Propulsion Laboratory’s (JPL’s) RPS flight 

acceptance profile. The workmanship profile amounts to 6.8 grms while the flight acceptance profile 

amounts to 8.7 grms, and represents the same level expected during launch. During convertor workmanship 

vibration, the NASASTD7001A workmanship profile will be applied in three orthogonal axes to the 

convertor alone. This test is intended to demonstrate that a hermetically sealed convertor in its final form 

has been assembled properly. Workmanship vibration will also be performed in three orthogonal axes on 

the generator once the convertors have been installed. Later, the generator will undergo a flight 

acceptance test, which will be at the same level expected for flight. Finally, the generator will undergo 

actual launch. Since the first mission to use ASRG has yet to be identified, the launch vehicle and 

spacecraft are unknown. However, the JPL RPS flight acceptance profile is based on historical launch 

vehicles and spacecraft that used RPS, and is intended to encompass all possibilities. 

The structure of the generator housing creates a particular vibration transfer function between the 

vibration source and the convertor. Thus, if a workmanship or flight acceptance profile is applied to the 

generator, the convertor will experience something different. To quantify this, LM performed vibration 

testing on the ASRG engineering unit !&E) (Meer (2009) and Chan (2008)). Accelerometers were 

placed at various locations during these tests, including most importantly on the alternator housing of the 

convertors. The response of this accelerometer was recorded when the generator was exposed to 

workmanship and flight acceptance profiles in all three axes. These tests provided profiles that could later 

be used to expose a convertor by itself to the same vibration environment it would experience when 

installed in the generator. The generator utilized two ASCEs, but the similarity to the flight design is 

sufficient that these data were applicable for generating the proper vibration profiles. This was necessary 

since there is only one generator engineering unit, and it is preoccupied with extended operation. No 

additional flight generator housing was available for vibration testing. However, there have been several 

stand-alone convertors, which were fabricated for developmental purposes, available for vibration testing. 

A fixture was designed to couple a stand-alone convertor to a vibration table. An example of a convertor 

installed in the fixture is shown in Figure 1. This fixture was designed to have high stiffness to yield high 

modal frequencies. This was done so that the transfer function between the table motion and the convertor 

would be as close to unity as possible. With this setup, the measured alternator housing response from the 

engineering unit testing can be used as the table control spectrum, and the convertor will experience the 

same vibration as if it were installed in the generator. The convertors can also be exposed to qualificationlevel vibration, which is defined as 3 dB above flight acceptance level. Increasing by 3 dB has the effect 

of increasing the average grms of a particular profile by a factor of 1.414 (square root of 2). More details 

about the vibration testing methods employed at Glenn can be found in Meer (2009). 

Launch Vibration Durability 

In 2009, a series of “durability” tests were conceived for the ASC-E2 engineering units that were 

being developed by Sunpower for Glenn. At this point in time, no non-hermetic ASC had been exposed to 

the vibration levels mentioned earlier. Thus, an in-depth internal inspection could not be completed to 

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indicated the possibility of a limited number of contact events, due to piston motion excursions. To 

address this, one of the durability tests conceived consisted of launch vibration on a convertor with a 

removable alternator housing that would permit inspection. The goal of this durability test was to quantify 

the effect of launch on the convertor’s internal components, such as the solid-lubricated surfaces, or the 
NASA/TM—2013-216544 3 

Figure 1.—Advanced Stirling Convertor installed in vibration test fixture on slip table. 

fasteners. The “vibration durability” test plan consists of three phases, the first of which has been 

completed. First, the convertor was exposed to the workmanship-level vibration tests in all three axes. 

This consisted of a total of six exposures for this phase, as in each axis there is a convertor-alone 

workmanship profile, and a profile for workmanship when installed in the generator. A disassembly and 

inspection of the convertor followed this phase. The second phase will consist of exposing the convertor 

to qualification-level vibration in the lateral directions only. For this case, there is only a generator-level 

vibration profile, as only the ASRG qualification unit (QU) will be subjected to qualification-level 

vibration. A disassembly and inspection of the convertor will follow this phase as well. Finally, the 

convertor will be subjected to qualification-level vibration in the axial direction. Again, this will be a 

profile to simulate what the convertor sees when installed in the generator. During this test the piston 

stroke will also be deliberately increased to induce a controlled number of contact events. The goal is to 

accumulate 30 contact events. This phase of the durability test is intended to address the possibility of a 

launch that was more violent than expected. The deliberate contact portion of this durability test will 

provide data that will allow assessment of the effect of such contacts. It should be noted that contact in 

this manner is with relatively low energy, as the reciprocating piston would be near a zero velocity at its 

end points of motion. 

ASC-K=

d prepared for the vibration durability test. The first phase of this test 

was conducted in June 2012. Prior to this, the convertor had undergone standard performance testing at 

Glenn, EMI characterization, and a centrifugal acceleration test (Meer (2012)). The convertor was 

returned to Sunpower for inspection after the first phase of vibration testing and all results were positive 

based on phase inspection of the fasteners and solid-lubricant surface. Prior to the test, the fasteners were 

marked with torque stripes. No fasteners showed signs of motion. The break-loose torque values for all 

fasteners were within 10 percent or their installation torques. There was no discernable change to the 

solid-lubricant surfaces. These data suggest the convertor design has sufficient margin to withstand 

workmanship-level vibration exposure. This is concluded because there was no fastener motion, and any 

piston/cylinder or displacer/cylinder contact did not generate surface wear nor debris. Phase 2 of the 

vibration durability test is scheduled to be completed in the spring of 2013. 
NASA/TM—2013-216544 4 

Heater Head Oxide Vibration Testing 

A series of vibration tests of ASC heater heads was initiated in June 2012. During post-production 

nondestructive evaluation of several heater heads, oxide inclusions were discovered. The effect of these 

oxide inclusions on the pressure containment capability of the heater heads was unknown, but the initial 

evaluation suggested they would not compromise the pressure boundary. A series of tests were planned 

and conducted to characterize the behavior of these inclusions and their suitability for use in ASC flight 

units. The tests consisted of exposing several of these heater heads to workmanship and qualificationlevel vibration, followed by inspection. The inspection consisted of a helium leak rate measurement to 

ascertain hermeticity, and examination of the oxide inclusion to look for signs of crack propagation. After 

evaluating all the stress states throughout the life and mission of the heater heads, these two vibration 

exposures were deemed the most severe and thought to be the best opportunity to induce crack 

propagation. A total of five heater heads were tested: heater heads E08, G02, G03, G05, and F06. These 

heater heads each contained several inclusions of varying length, depth and position, and thus were a 

good sampling of nonconforming heads. 

Since only a subcomponent of the convertor was going to be tested, mounting fixtures had to be 

designed to simulate the loads the heater heads would experience when configured as part of the 

convertor, and to allow the heater heads to be installed into the existing vibration test fixture (described 

previously). The most desired configuration was to use as many ASC production parts as possible to best 

represent the connection of the heater head to the source of vibration. Spare ASC parts were assembled to 

form an “ASC mockup.” This mockup included all of the ASC structural components and the internal 

alternator, but none of the moving internal parts. This configuration was used on testing two of the units 

(E08 and G05), and is shown in Figure 2 (left). 

 

Figure 2.—Advanced Stirling Convertor (ASC) mockup with G05 (left) and ASC dynamic simulator with 

G02 (right). Heater heads are circled. 
NASA/TM—2013-216544 5 

Due to the limited number of ASC spare parts, another fixture had to be designed and manufactured 

for the remaining heater heads. This fixture, deemed the “ASC dynamic simulator,” was designed to 

attach to the vibe test fixture and simulate the heater head mechanical interface and dynamic loading. This 

configuration is shown in Figure 2 (right). The ASC dynamic simulator was used for testing heater heads 

G02, G03, and F06. All of the heads were pressurized to the proper charge, installed in the standard ASC 

vibration test fixture, and separately tested to qualification-level random vibrations. 

For this testing new instrumentation was installed that had not been included in previous Stirling 

convertor vibration tests. This included an accelerometer mounted on the collector of the heater head and 

strain gages mounted on the heater head. These were not included in previous testing because they would 

normally be in proximity to a hot component of the convertor. The heater head tests were conducted at 

room temperature as the area of highest stress is at the coldest part of the head, and analysis indicated the 

heater was not necessary. With the heater head at ambient temperature, these instruments could be added. 

This instrumentation was necessary to determine the amount of stress the inclusions were subjected to 

during testing, and how well the ASC dynamic simulator matched the ASC mockup. All of the tests 

included the heater head accelerometer, but due to the timeline for testing, strain gages were not included 

on the first three tests. Figure 3 shows the location of the heater head accelerometer and strain gages. 

Accelerometers were placed on the alternator housing and cold-side adapter flange (CSAF) for all of 

these tests, and these are shown in Figure 4. 

Figure 3.—Location of heater head accelerometer and strain gages. Heater head portion is circled. 
NASA/TM—2013-216544 6 

Figure 4.—Photograph of the Advanced Stirling Convertor (ASC) mockup installed in the vibration test 

fixture and locations of accelerometers. 

TABLE I.—SUMMARY OF OXIDE INCLUSION HEATER HEAD TESTING AT QUALIFICATION-LEVEL VIBRATION 

E08 G05 G02 G03 F06 

Test date 6/19 8/22 9/13 9/28 8/23 9/05 9/06 9/25 

Axes All All Z-axis 

(lateral) 

Z-axis 

(lateral) All All All Z-axis (lateral) 

Post-test leak 

check Pass Pass Pass Pass Pass Pass Pass Pass 

Success was defined by surviving the vibration exposure without crack propagation nor helium 

leakage. This was tested by performing a pre- and post-vibration exposure hermeticity test with the same 

equipment and methods used to test flight hermetic convertors. All of the tested heater heads passed the 

hermeticity check. After the initial round of testing, head G05 was subjected to two more qualificationlevel vibration exposures, and heater head F06 was subjected to one more exposure. A summary of testing 

is shown in Table I. 

All of the test articles passed the vibration exposures, as none developed leaks, and no crack 

propagation was induced. No cracks were initiated even in the case of the G05 article, which was exposed 

to qualification-level vibration three times. It was also concluded from this testing and evaluation that the 

oxide flaws do not behave as cracks, and thus do not have the potential to propagate like separated base 

material would. The data from the accelerometers and strain gages were used to improve the finite 

element models that were previously constructed to predict oxide flaw behavior. These results were then 

used to guide selection criteria for the remaining flight heater head builds. The flaw size acceptance 

criteria were revised, thus making several previously suspect heater heads eligible. With this, the project 

will have an ample supply of heater heads to support the flight unit builds. 

There were several lessons learned via examination of data from the new instrumentation used for 

these tests. The vibration test fixture has features to apply a preload to the heater head, which is intended 

to replicate the use of a radioisotope heat source. The mechanism uses Belleville washers to effect an 

adjustable spring load. Upon installation, during which strain gages were present, the strain gage readings 
NASA/TM—2013-216544 7 

were higher than expected. This suggested that the applied force was actually higher than anticipated. The 

spring stack stiffness was measured using onsite laboratory equipment and found to be 30 percent higher 

than the manufacturer’s published data. This meant that the first four tests to be conducted were with a 

higher preload value than originally thought. Because of this, future use of these types of spring washers 

will require measured stiffness data to set the preload, rather than only manufacturer’s data. 

Another lesson learned was in the area of preload geometric alignment. It is desired to have a uniform 

axial load applied to the heater head by the preload mechanism. During installation of the preload on the 

heater heads with strain gages, it was found to be a difficult process to apply preload coaxial with the 

convertor’s axis. The standard installation procedure did not result in uniform loading. A sequence of 

tightening and loosening the preload bolts while monitoring at the strain gage readings was required to 

achieve a uniform, coaxial load. If needed for future testing, the loading mechanism of the vibration test 

fixture should be reexamined. Pre- and post-test sine sweeps showed a shift of the heater head resonant 

frequency in some cases. This indicated that something was changing within the fixture during vibration. 

Initially, this was thought to be caused by a broken ceramic washer used in the preload path. This washer 

was replaced by a metal washer, which reduced the magnitude of the natural frequency shift, but did not 

eliminate it. A more in-depth investigation needs to be conducted to understand the cause of this shift 

during testing. 

Conclusions 

Two unique structural dynamics tests have been conducted at NASA Glenn Research Center in 

support of the Advanced Stirling Radioisotope Generator (ASRG) towards flight. The first test consisted 

of exposing a convertor to workmanship-level vibration. The effect of this vibration on the internal 

components was quantified via inspection, which had not been possible with previous convertors. The 

results of the inspection demonstrated sufficient margin in the Advanced Stirling Convertor (ASC) design 

to withstand workmanship-level vibration. During follow-on test phases, this convertor will also undergo 

qualification-level vibration in the lateral directions and qualification-level vibration with induced piston 

contacts. These phases of the test will further determine design margin for qualification-level vibration 

exposure, as well as quantify the effect of a limited number of moving component contacts. ASC heater 

heads with oxide inclusions were during relevant vibration conditions. The results of these tests suggest 

that even the largest inclusions did not fail or propagate cracks when exposed to qualification-level 

vibration. These data suggest that heater heads with this type of flaw can still be used for flight. 

References 

Chan, J. (2008): Development of Advanced Stirling Radioisotope Generator for Planetary Surface and 

Deep Space Missions. AIAA20085768. 

Meer, D.W. (2009): Advanced Stirling Convertor Dynamic Test Approach and Results. AIAA2009

4552. 

Meer, D.W. (2012): Advanced Stirling Convertor Durability Testing: Plans and Interim Results. AIAA

20124249. 

Schreiber, J.G. (2006): Developmental Considerations on the Free-Piston Stirling Power Convertor for 

Use in Space. AIAA20064015. 

Wong, W.A. (2012): Pathfinding the Flight Advanced Stirling Convertor Design With the ASCE3. 

AIAA20124251.